Association between mineral-vitamin bolus supplementation during the breeding period and reproductive outcomes in dairy cows

The period between calving and first artificial insemination (AI) is characterised by major metabolic adaptations that may influence subsequent reproductive performance in dairy cows. In this retrospective matched cohort study, we examined the association between mineral-vitamin bolus supplementation during the early breeding period and reproductive performance and blood biochemical parameters under commercial dairy farm conditions. Clinically healthy dairy cows were enrolled in matched pairs based on calving date and lactation number. Within each pair, one cow received the bolus, whereas the other served as a control. Allocation was not randomised. A total of 88 cows were included in the final analysis (45 supplemented and 43 control cows). Cows received two boluses at 51.13 ± 3.15 days in milk (DIM) and two additional boluses 14 days later. Each two-bolus dose provided a declared daily delivery of 2.4 g magnesium, 130 mg zinc, 36 mg copper, 1.5 mg selenium, 132,000 international units (IU) of vitamin A, 500 IU vitamin E, and 300 mg β -carotene over a 10-day release period. Reproductive outcomes included conception at first AI, pregnancy by 150 and 300 DIM, time to confirmed pregnancy, days to first oestrus, and days to first AI. Blood samples were collected at enrolment and 14 and 21 days later. After adjustment for baseline metabolic status, parity, and DIM at enrolment, bolus supplementation was associated with higher odds of conception at first AI (odds ratio (OR) = 3.45; 95% confidence interval (CI): 1.07–11.11; p = 0.038) and, as an exploratory endpoint, pregnancy by 300 DIM (OR = 3.80; 95% CI: 1.19–12.19; p = 0.025). No differences were observed in pregnancy by 150 DIM or time to confirmed pregnancy. Supplementation was not associated with the resumption of oestrus overall. An earlier resumption in multiparous cows was an exploratory subgroup finding. Supplemented cows had higher serum magnesium concentrations and lower lactate dehydrogenase (LDH) activity than controls (supplementation-by-time interaction, p < 0.001 and p = 0.025, respectively). Bolus supplementation was therefore associated with higher odds of conception and pregnancy and with altered mineral status. The difference in cumulative pregnancy rates emerged only after 180 DIM, more than 100 days after supplementation ended.